Graz team sheds new light on dark states, another Austrian breakthrough in quantum physics

Light shines through opaque materials if certain conditions are met. By establishing the existence of another such condition, scientists at the Graz University of Technology have created additional possibilities for using light to control revolutionary new optical components. The project, funded by the Austrian Science Fund, has experimentally demonstrated an effect previously only known from theoretical calculations.

It adds another impressive achievement to the recent run of success marked up by Austrian quantum physicists, one that would not have been possible without long-term support from the FWF.

Materials absorb light when their electrons react to the frequency of light waves. The light energy is transformed by the material into kinetic energy. However, if the oscillations of the electrons are suppressed the light proceeds on its way unimpeded, the material becomes transparent.

'One way of suppressing the oscillations is irradiating the target with two different laser beams simultaneously,' said Professor Laurentius Windholz of the Institute for Experimental Physics at Graz University of Technology. 'Each laser beam on its own is capable of being absorbed, but irradiation with both translates the atoms into a state in which the electrons can react to neither the first nor the second beam.'

This effect, known as electromagnetically induced transparency, depends on the frequency (colour) and intensity of the light waves. It had previously been calculated that in certain cases the radiation phase, the relative position of the wave troughs and peaks, would influence it. Professor Windholz's team has now provided the first experimental proof that this is so.

The researchers used systems with three or four lasers operating at different frequencies. The laser beams passed through a cell filled with sodium vapour, which normally absorbs light at a wave length of 596 nm. Absorption was significantly reduced when the cell was simultaneously irradiated with four laser beams of slightly different frequencies. Windholz was also able to show that the effect was influenced by the relative phase of the light waves.

'What we are investigating here are very fundamental aspects of a technology that has yet to be developed,' said Windholz of his work. 'If we are to replace electronic components by purely optical and hence faster ones, we will need a better understanding of the interactions between light and matter. By exploiting EIT it may be possible in future to replace electrical switching elements like those used in computer chips by optical switches which are faster.'

The breakthrough is just one of a series of success stories that have recently underpinned the excellent international reputation of Austrian quantum physics. The past nine months have also seen two cover stories in British science magazine Nature*, and the appointment of Austro-Hungarian physicist Ferenc Krausz as director of the Max Planck Institute for Quantum Optics in Germany.

Physicist Dr. Laurenz Niel who heads the public relations department at the FWF sees these achievements as proof of the effectiveness of the Fund's strict selection criteria for projects. The quality of the research is the prime consideration. Niel commented: 'In the past three years we have supported over 45 quantum physics projects. You can imagine how pleased, and proud, we are to have contributed to the international standing of an interesting field of Austrian fundamental research in this way.'